>>>> - When the window is resized the divider is reset to the center
>>>> position. I would expect the divider to keep its relative position
>>>> (e.g. one third to the left) even when the window is resized.
...
>> It seems like the re-positioning of the divider lags one step behind
>> the actual resizing. Try making the window smaller, then larger, and
>> then larger again, and notice the movement of the divider. :-)
> And what should I notice? Works perfectly fine for me.
> So what doesn't work?

I'll try to explain what happens step by step, on a newly opened non-maximised TortoiseMerge window (E=Expected behavior, A=Actual behavior):
1.) Window is made smaller
- E: Divider should move to the left
- A: Divider does not move
2.) Window is made larger
- E: Divider should move to the right
- A: Divider moves to the left
3.) Window is made larger
- E: Divider should move to the right
- A: Divider moves to the right

...and so on. The actual behavior of the divider in a resize is the expected behavior of the divider in the previous resize.
Quite interesting! It took a while for me to notice what was going on. When doing random size adjustments, the behavior of the divider looks equally random. :-)